The present invention relates to an optical recording medium and a process for producing the same. More in detail, the present invention relates to an optical recording medium produced by irradiating a laser beam on a recording layer to heat locally for forming an ablative hole or a depression in the thus heated part, thereby recording informations, and to a reliable process for well-reproductively producing the optical recording medium.
As an optical recording medium produced by irradiating a laser beam to a thin recording layer formed on a substrate, thereby forming a hole, a depression or a protuberance thereon, it has been hitherto known to use thin Te films. Since Te is large in the light absorption coefficient, melts at a low temperature and is low thermal conductivity, Te shows a high sensitivity in the recording by the above-mentioned method. However, there is a problem that Te films tends to be oxidized rapidly in air, the degradation of the light absorption efficiency by oxidation results the degradation of recording sensitivity.
As the medium in which the degradation resistance of Te films has been improved, those using an alloy containing Se other than Te, those using lower oxides of Te, those using an organic polymer layer in which Te is dispersed, etc. have been known [for instance, refer to Japanese Patent Applications Laid-Open (KOKAI) No. 53-31104(1978), No. 58-54338(1983) and No. 57-98394(1982)].
Although, the above-mentioned recording medium is produced by a vacuum evaporation method or an ion-plating method, a sputtering method is preferably adopted because of the favorable controlability during the deposition of films.
As a result of the present inventors' studies on the films produced by sputtering of the Te or Te based materials using pure argon gas, it was found by X-ray diffraction, electron diffraction and the transmission electron microscopy that the large crystal grains of a size of from several thousands .ANG. to several .mu.m are observed in the whole area of these films, and that the flatness, the shape of pits and the recording sensitivity are poor and a large amount of noise is generated in a readout signal. In addition, it has been made clear that the polycrystalline structure of the deposited films is unstable and accordingly, since the reflectance increases nearly to 1.3 times as compared to the initial reflectance within 24 hours in the accelerated test at a temperature of 65.degree. C. and a relative humidity of 80%, the stability of such a recording medium in the course of time is extremely poor.
For solving the above-mentioned problem, there is a method by which the film of recording layer has non-crystalline or microcrystalline structure and the temperature at which the above-mentioned micro-structures are transformed into the polycrystalline structure of larger grain size, that is the crystallization temperature, is made to be higher, thereby stabilizing the micro-structure of the films at room temperature. Concretely, it may be exemplified that a thin recording layer of a Te based alloy containing Ge, Pb, Sn, etc. it used [refer to Japanese Patent Publication No. 59-35356(1984)].
Furthermore, it is proposed that the same effect as above is obtained even by dispersing Te in an organic substance through the reactive sputtering [refer to Japanese Patent Applications Laid-Open (KOKAI) No. 57-165292(1982) and No. 57-78394(1982)].
However, even in the produced medium by the above-mentioned process, the change in reflectance (transmission) of the medium in the course of time occurs with the change of the micro-structure and the degradation of the medium by the long-term irradiation of readout laser light. Namely, it has been difficult to maintain the micro-structure of the recording layer in a stable state for a long-time period in the case of using a Te based alloy film as the recording layer.
On the other hand, in the optical recording medium wherein the ablative holes or the depression are formed as the pits for recording the informations, not only the recording layer but also the state of the interface between the recording layer and the substrate or the underlayer is important as the primary factor which determines the laser beam power required for forming the pit, namely, the recording sensitivity and the forms of pits.
In order to form the pits in the thin layer of the recording medium comprising the above-mentioned substrate and the thin recording layer by the laser beam, it is necessary that the materials of the recording layer which is melted locally by laser heating removed from the substrate while overcoming the work of adhesion of the film to the substrate. For the purpose of reducing the adhesion and of improving the recording sensitivity, a disposition of an underlayer comprising a thin layer of fluorocarbon polymer between the recording layer and the substrate has been examined [refer to Japanese Patent Application Laid-Open (KOKAI) No. 59-90246(1984)]. The factors contributing to the adhesion of the films to the substrate are the surface tensions of the recording layer and the substrates thereof, the molecular weight and the degree of crosslinking of the surface layer of the substrate, etc. As the work of adhesion of the recording layer to the substrate is smaller, the pit can be formed in a shorter pulse width by a smaller laser beam power. The above-mentioned fact means the improvement of the recording sensitivity, and therefore the recording of high speed and the use of a cheap semiconductor laser diode of a low output power become possible. However, in order to perform a recording of a higher quality, it is required that the sensitivity is improved but also that not only recorded pits have sharp and well-defined edges are uniform.
On the other hand, to the optical recording medium, it is required that the storage capacity is large, namely that a recording of high density is possible, in addition to the above-mentioned specific properties. In order to improve the storage capacity of the optical recording medium of the perforating type, it is required that the minimum size of the pit is as small as possible. In the case where due to the large heat conductivity of the recording layer, the region to be melted and removed by irradiation of laser beam becomes too large and in the case where due to the smallness of the adhesion of the recording layer to the underlayer, the amount of the substance to be removed becomes too large and the size of the pit is apt to be enlarged, and accordingly in such a case, high density storage is impossible.
Furthermore, in the above-mentioned media, since there is a tendency that the size of the pit changes sensitively by the slight change of the laser beam power, the stable and accurate recording of the digital signals is difficult.
In the case where a thin film of fluorocarbon polymer is provided as the underlayer, it is relatively easy to improve the recording sensitivity, however, there still remains problems concerning the above-mentioned shape and size of a pit. In Japanese Patent Application Laid-Open (KOKAI) No. 59-90246(1984), any method for dissolving the above-mentioned problem concerning the pit shape has not been given.
Furthermore, in addition to the problem concerning the above-mentioned pit shape, in the case of recording by the laser beam of short pulse width or in the case where the disk is rotated at a high constant angle velocity, since particularly in the outer region of the disk, the energy density of the laser beam focused on the unit area of the surface of the optical recording medium is small, the laser beam output necessary for forming a pit is larger, and the requirement for the improvement of the sensitivity to the optical recording medium is more severe than that of the inner region.
In order to fulfil the above-mentioned requirements, the combination of the material of the recording layer and that of the substrate or the underlayer becomes an extremely important factor. Namely, in order to shorten the length of the minimum size of pit, it is desirable that the adhesion is larger, and on the other hand, in order to improve the sensitivity, it is desirable that the adhesion is smaller. In other words, the two requirements which mutually contradict at a glance must be fulfilled. In order to overcome the above-mentioned contradiction, for instance, a method of utilizing an organic compound which decomposes and/or sublimes at a low temperature while having a high adhesion (nitrocellulose, guanine and pigments such as phthalocyanine) may be mentioned (refer to the Proceeding of XXXII Combined Recture Meeting of Applied Physics, p. 115, Spring in 1985). However, by such a method, a sufficient sensitivity and stability of the optical recording medium have not necessarily obtained. Furthermore, the physical properties of these existing organic compounds (decomposition temperature, sublimation temperature and adhesion) are specific to each of them, and it is impossible to optimize the properties of those compounds easily and flexibly corresponding to the combination of the various recording layers and the driving system.
Moreover, the above-mentioned sublimative pigments cannot be formed into a thin layer by the sputtering method and the plasma polymerization method and accordingly the constitution of a consistent dry process with forming that of the recording layer while using the sputtering method is impossible.
As a result of the inventors' further studies, it has been found that by carrying out a reactive sputtering in gaseous mixture of a selenium fluoride gas and argon gas while using an alloy containing Te and Se as a target material, the obtained optical recording medium is excellent in the recording sensitivity, the pit shape, the smoothness of the surface of the recording layer thereof, the archival stability etc. and shows the low readout noise, and based on the finding, the present invention has been attained.